New Insight from the James Webb Space Telescope on Variable Active Galactic Nuclei

Abstract

Variability detected in galaxies is usually attributed to their active galactic nuclei (AGNs). While all AGNs are intrinsically variable, the AGN unification model predicts that type~2 AGNs rarely vary because their engines are blocked by dust tori. Previous UV-to-near-IR variability studies largely support this expectation. Here, we present a variability study by James Webb Space Telescope (JWST) that reveals a more subtle picture. Using NIRCam imaging data from three surveys over $\sim$140~arcmin$^2$ in the COSMOS field, we found 117 galaxies with $\geq 4$$\sigma$ variability in the F356W band across $\sim$2-year baseline. Cross-matching with the existing JWST spectroscopic data, we identified five of them at $z=0.19$--3.69 (F356W corresponding to rest-frame $\lambda\approx0.76-2.97$~$\mu$m), which were all coincidentally observed by a NIRSpec program almost contemporaneously with the last imaging epoch. One additional variable was identified at $z=0.90$ using the archival Keck telescope data. These six objects form our spectroscopic subsample. Interestingly, two reside in close-pair environments, while two others form a close pair themselves. Most of their light curves can hardly be explained by nuclear transients, and AGN variability is a more plausible cause. However, among these six objects, (1) only one shows broad Bracket and Pfund series permitted lines ($\Delta v > 1000$~km~s$^{-1}$) indicative of a type~1 AGN; (2) two show narrow permitted lines (H$\alpha$ and/or He~I$\lambda10830$) consistent with type~2 AGNs, with another one likely type~2 based on the host galaxy properties; and (3) two others, which form a pair, show no emission lines. Our results add more challenges to the unification model.